视频中的 screen，cpu 和mem 都是private，为何只有screen需要用set 和get 方法获取，其他两个为何可以直接使用？（代码如下）

Public class Telephone(){

  private float screen;

  private float cpu;

  private float mem;

  public float getScreen(){

     return screen;

  } 

  public void setScreen(float newScreen){

     screen=newScreen;

  }

  public Telphone(){

     System.out.println("无参的构造方法执行了！“）；

  public Telphone(float newScreen,float newCpu,float newMem){

     screen = newScreen;

     cpu=newCpu;

     mem=newMem;

 }

public static void main(String[]args){

    Telphone phone = new Telphone();

    Telphone phone2 = new Telphone(5.0f,1.4f,2.0f);

    //phone2.cpu = 6.0f;

    phone2.setScreen(6.0f);

    System.out.println("screen:"+phone2.getScreen());